Airlines should continue their ban on the use of mobile phones on board aircraft because of possible interference with navigation and communication equipment, according to a study published Friday by the U.K. Civil Aviation Authority.A series of tests exposing a set of aircraft avionic systems to simulated cell phone transmissions revealed various adverse effects on the equipment's performance, CAA said in a statement. Although the equipment allowed a margin above the "original certification criteria for interference susceptibility," the margin wasn't sufficient to protect against potential cell phone interference under worst-case conditions, the authority said.Vodafone Group PLC participated in the tests.The study could deal a blow to some airlines, such as Scandinavian Airlines System AB (SAS), which hope some day to offer passengers the opportunity to use their mobile phones in the air in much the same way they're accustomed to on the ground.Cell phone use has long been banned on airplanes, while the use of many other electrical devices such as notebook computers is banned on take-off and final approach. But passengers, particularly businesspeople, are interested in using their phones on planes to keep in touch with their offices and customers.In the U.S., the Federal Aviation Administration (FAA) bans the use of cell phones on airplanes because, like the CAA, it too is concerned about wireless calls interfering with a plane's navigation system.From March 1996 to December 2002, CAA recorded 35 aircraft safety-related incidents that were linked to cell phones, the authority said.The reported interference incidents included interrupted communications due to noise in the flight crew's headphones, according to the study.CAA recommends a continued ban on mobile phone use by passengers in aircraft and urges airlines to introduce safety procedures that ensure phones are switched off.Whether the CAA study will encourage airlines to prohibit the use of mobile phones with flight-safe features remains to be seen.Last week, SAS announced a policy for allowing passengers during flights to use all mobile phone functions, such a calendars, address books and reading e-mail, that require no signal transmission. To do so, passengers require phones equipped with a flight-safe mode, which essentially prevents a handset from sending or receiving signals required to make phone calls.Nokia with its 9210i Communicator and Sony Ericsson Mobile Communications AB with its P800 smart phone are among the first manufacturers to offer handsets with the flight-safe feature."We continue to recommend to our customers that they should turn off their mobile phones when inside aircraft and should only turn them back on if equipped with the flight-safe mode," said Nokia spokesman Damian Stathonikos. "But we understand that some airlines prefer passengers not to use their mobile phones at all."